BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



185 



that foods, in general, contain far less calcium than the animal body 

 (Table 8-1). Milk is one of the best food sources, a quart supplying 

 approximately one gram of both calcium and phosphorus. Cheese, egg 

 yolk, and green leafy vegetables are also excellent sources; most other 

 vegetables contain moderate amounts. Ground bone is well utilized by 

 the animal body as a source of calcium and phosphorus, and, in fact, 

 bone meal is commonly used as a supplement to the feed of domestic 

 animals. The bones in most types of canned fish should be regarded as 

 valuable food rather than being carefully picked out and thrown away 

 as is usually the case. 



On the other hand, the phosphorus supply needs no special attention. 

 A diet providing the recommended amounts of protein and calcium will 

 almost certainly contain sufficient phosphorus, at least for human beings. 

 Cattle, however, occasionally become phosphorus deficient, but only 

 when their feed consists almost entirely of forage grown in certain areas 

 where the soil is low in this element (e.g., parts of Wisconsin, Montana, 

 Texas, and Florida). 



Diets inadequate or imbalanced in their calcium, phosphorus, or vita- 

 min D contents lead to rickets in children and to osteomalacia (softening 

 of the bones) in adults (see chapter on Vitamins). All three factors are 

 also obviously essential for sound, well-formed teeth, although many other 

 influences likewise play important roles in dental health. 



Magnesium. This element is also an indispensable constituent of all 

 living cells. Since it is a part of the chlorophyll molecule (p. 388), 

 magnesium is essential for photosynthesis and, hence, indirectly respon- 

 sible for the production of all our foodstuffs. About three-fourths of 

 the magnesium in the animal body is contained in the skeleton, the rest 

 being present in the blood and other body fluids as Mg++ ions. The 

 normal concentration of Mg++ in human blood serum is 2-3 mg. per 

 100 ml. Marked increases of up to 10-20 mg., resulting, for example, 

 from the injection of soluble magnesium salts, lead to generalized anes- 

 thesia, complete muscle relaxation, and eventual death. If animals or 

 human beings are deprived of magnesium until the blood level falls to 

 0.6-0.8 mg. per 100 ml., or less, symptoms of magnesium deficiency 

 appear. These include dilation of capillaries, extreme nervousness, con- 

 vulsions (tetany) , and death. 



Magnesium ions are necessary for the normal activity of several 

 enzymes in the body, particularly peptidase, carboxylase, enolase, hexo- 

 kinase, and others. In some cases the Mg++ may be replaced by other 

 ions, such as Mn+ + . However, for many biological functions there 

 exists marked antagonism between various inorganic ions. Calcium and 

 magnesium offer an outstanding illustration of this antagonism. Thus 

 the enzyme adenosine triphosphatase, which is activated by Ca++ ions, 

 is inhibited by Mg+ + . Likewise Ca++ is the best antidote for the 



